Process of preparing an iron base melt



United States Patent PROCESS or PREPARING AN IRON BASE MELT Paul L. Jackson, Dearborn, and Victor F. Zackay, Detroit, MiClL, assignors toFord Motor Company, Dearborn, Mich., a corporation of Delaware No Drawing. Application September 2, 1954, Serial No. 453,954

3 Claims. (Cl. 75-1305) This invention resides in the metallurgical field and is more particularly concerned with a series of ferrous alloys designed to make available excellent high temperature physical properties without recourse to quantities of highly strategic materials and to enable such ferrous alloys to be produced Without resort to any of the socalled supercritical elements.

The usual ferritic alloys become worthless at temperatures in the neighborhood of 1200 F. For service at temperatures exceeding 1200 F., recourse is bad to the predominantly austenitic alloys as exemplified by the conventional alloys of the 188 type of stainless steel. These predominantly austenitic alloys have excellent physical properties above 1200 F. but their use is restricted due to the inclusion of substantial quantities of critical, expensive and scarce elements.

It is a particular object of this invention to provide an economically feasible family of alloys usable in the temperature range up to 1500 F. This range is sufi'iciently extended to enable the use of this type of material for many applications now requiring the more expensive predominantly austenitic alloys.

This family of alloys is of course predicated upon an iron base and includes as necessary adjuvants the austenite forming elements, manganese and nitrogen. Within limits which Will be readily apparent to the skilled metallurgist, the common alloying additions used in the ferrous art such as carbon, silicon, nickel, molybdenum, tungsten, vanadium, columbium, titanium, etc. may be employed so long as the basic balance set up in the original iron-manganese-chromiurn-nitrogen alloy is not destroyed.

The metallurgical industry has made extensive use in the last two decades of nitrogen as an alloying element to gain the benefits of its intense austenizing power, ready availability and low cost. Nitrogen is further advantageous in that it imparts hot strength, impact resistance and is a potent grain refiner. The following United States Letters Patent are made of record as demonstrating the decided interest in this type of alloy: 1,990,589, 2,056,766, 2,071,740, 2,120,554, 2,140,905, 2,191,790, 2,197,955, 2,198,598, 2,199,096, 2,212,496, 2,294,412, 2,302,607, 2,657,130, 2,671,726, 2,686,116.

We have discovered that iron base alloys containing to 30 per cent chromium and 10 to 20 per cent manganese may be greatly improved by the addition of 0.3 to l per cent nitrogen, the ratio of chromium to nitrogen being less than 70 to 1 and ranging downwards to 25 to 1. To enable a ratio of chromium to nitrogen of this order to be obtained, it is necessary that the addition of nitrogen be made under superatmospheric pressure. The retention of nitrogen is enhanced by causing rapid solidification of the casting. The melt should be maintained under a pressure of at least one atmosphere higher than ambient. Such pressure may be obtained from a compressed atmosphere of nitrogen or other gas inert to the melt, or it may be attained by rotating the melt sufficiently rapidly to multiply the effect of gravity to the desired extent. Under rotating conditions an inner annulus of an inert 2,745,746 Patented May 15, 1956 'ice slag is helpful in applying pressure to the sublatent melt. Sufiicient pressure may be so applied to permit solidification directly without transferring to a further mold and a sound casting obtained. It is preferable that the ingots so cast be restricted in size to permit rapid solidification and if necessary cast in cooled molds to expedite solidification. It is axiomatic that having attained the desired high chromium to nitrogen ratio the melt be maintained continuously under superatmospheric pressure until solidification has occurred. In computing the chromium nitrogen ratio, the nitrogen combined with elements such as zirconium and titanium which form very stable nitrides should be ignored since such nitrogen is firmly combined and not available as an austenite former.

Only by observing these precautions will an ingot be obtained which meets the commercial specifications for freedom from gas defects.

T o suit any given commercial requirements, the basic iron-chromium-manganese-nitrogen alloy may be modified by any desired amount of carbon, by the addition of silicon up to 3 per cent, by the addition of nickel up to 8 per cent, by the addition of molybdenum or tungsten up to 6 per cent or vanadium up to /z of l per cent. As those skilled in the art will readily appreciate, the original iron-chromium-manganese-nitrogen alloy will be modified by these additions either towards the alpha or gamma form. The addition of carbon and nickel, both austenite formers will tend to make the alloy more stably austenitic and permits a modification of the original alloy towards the low manganese side since carbon or nickel can replace part of the manganese. Similarly the addition of a strong ferrite former such as silicon or the equal ferrite formers such as tungsten or molybdenum will require a concomitant increase in some or all of the austenite formers such as carbon, manganese, nickel and nitrogen.

The following tabulation is presented as illustrative of alloys prepared during the perfection of this invention.

TABLE I Composition of alloys [Oompc-sition, weight percent] Element Alloy Number Cr Mn Mo Ni N Si C 17. 0 l1. 8 l. 90 0. 30 0. 01 0. 01 15. 9 12. 3 2. 02 07 46 0. 25 0. 03 17. 0 12V 7 2. 50 0. 44 07 22 0. 04 15. 9 l3. 7 1. 79 0. 44 07 26 0. 04 15. 7 l5. 4 l. 94 1. 45 0. 40 l. 42 0. 08 15v 6 l5. 2 lv 94 45 0. 39 1. 08 0. 07 16. 7 15. 7 5. 8 0. 62 0. 23 0. 06

Alloys 46, 48 and contained minor amounts of alpha iron. The remainder of the alloys were completely austenitic.

Attention is specifically invited to alloys 47 and 49. At a temperature of 1250 F. these alloys exhibited physical properties equal to the best of the so-called super iron base alloys. These alloys were compared with an alloy designated 316 which is an 18-8 type alloy with 2 per cent of molybdenum and with an alloy designated 16-25-6 which is an alloy containing 16 per cent chromium, 25 per cent nickel and 6 per cent molybdenum. At 1250 F. alloys 47 and 49 were the full equivalent of these standard and highly critical alloys.

It will be noted that Jennings 2,657,130 teaches in column 5, sample P, an alloy containing 0.530 nitrogen and 20.43 per cent chromium. It would appear at first blush that this alloy falls d rectly within the range of chromium, nitrogen ratios herein specified. However it is applicants invariable experience that any attempt to prepare such an alloy having-a chromium-nitrogen ratio less than 70 to 1 results in a gassy unsound ingot if it is not held and cast under super-atmospheric pressure and quickly cooled.

The particular form in which the nitrogen is added to the alloy is not critical and any convenient form of this element may be used. For example, the nitrogen may be added as molecular nitrogen or it may be derived from such materials as cyanides, cyanamides, ammonia or nitrogen bearing alloys such as high nitrogen iron or high nitrogen ferro-chrome.

We claim as our invention:

1. The process of preparing an effectively gas free iron base casting comprising preparing an iron base melt containing as essential alloying ingredients chromium in an amount from to 30 per cent, and manganese from 10 tog percent, subjecting said melt to a superatmospheric pressure amounting to at least one atmosphere above the ambient, alloying the melt under said pressure of at least two atmospheres with nitrogen in an amount from 0.3 to 1 per cent, and until the ratio of chromium to nitrogen is between 70 to l and to 1, casting the nitrogenous melt into a mold without interrupting the application of pressure, and abstracting heat from the melt rapidly enough to cause solidification without substantial ebullition of gas.

2. The process of preparing an effectively gas free iron base casting comprising preparing an iron base melt containing as essential alloying ingredients chromium in an amount from 10 to per cent and manganese from 10 to 20 per cent, subjecting said melt to the superatmospheric pressure of an inert atmosphere amounting to at least one atmosphere above ambient, alloying the melt under said pressure of at least one atmosphere above ambient with nitrogen in an amount of 0.3 to l per cent and until the ratio of chromium to nitrogen is between to l and 25 to l, casting the nitrogenous melt into a mold without interrupting the application of pressure and abstracting heat from the melt rapidly enough to cause solidification without substantial ebullition of gas.

3. The process of preparing an effectively gas free iron base casting comprising preparing an iron base melt containing as essential alloying ingredients chromium from 10 to 30 percent and manganese from 10 to 20 per cent, subjecting said melt to a superatmospheric pressure amounting to at least one atmosphere above ambient by centrifugal force, alloying the meltunder said pressure with nitrogen in an amount of 0.3 to 1.0 per cent and until the ratio of chromium to nitrogen is between 70 to 1 and 25 to 1, and permitting the nitrogenous melt to solidify under sufiicient centrifugal pressure to prevent substantial ebullition of gas.

Arness Nov. 26, 1935 Tanczyn Dec. 7, 1954 

1. THE PROCESS OF PREPARING AN EFFECTIVELY GAS FREE IRON BASE CASTING COMPRISING PREPARING AN IRON BASE MELT CONTAINING AS ESSENTIAL ALLOYING INGREDIENTS CHROMIUM IN AN AMOUNT FROM 10 TO 30 PER CENT, AND MANGANESE FORM 10 TO 20 PERCENT, SUBJECTING SAID MELT TO A SUPERATMOSPHERIC PRESSURE AMOUNTING TO AT LEAST ONE ATMOSPHERE ABOVE THE AMBIENT, ALLOYING THE MELT UNDER SAID PRESSURE OF AT LEAST TWO ATMOSPHERES WITH NITROGEN IN AN AMOUNT FROM 0.3 TO 1 PER CENT, AND UNTIL THE RATIO OF CHROMIUM TO NITROGEN IS BETWEEN 70 TO 1 AND 25 TO 1, CASTING THE NITROGENOUS MELT INTO A MOLD WITHOUT INTERRUPTING THE APPLICATION OF PRESSURE, AND ABSTRACTING HEAT FROM THE MELT RAPIDLY ENOUGH TO CAUSE SOLIDIFICATION WITHOUT SUBSTANTIAL EBULLITION OF GAS. 